The present invention is a method for converting a sample of particulate silicon into monocrystalline silicon that is suitable for analysis of low-level contaminates present in the particulate silicon. The method uses a silicon vessel to retain the particulate silicon. The silicon vessel, containing the particulate silicon, is float-zone processed into monocrystalline silicon. The resultant monocrystalline silicon can then be analyzed by standard means for elemental impurities.
The production of semiconductor material for use in rectifiers, transistors, photo transistors, and the like requires extremely pure monocrystalline silicon. To control the quality of the monocrystalline silicon it is important to be able to assess the level of contaminates in the polycrystalline silicon from which the monocrystalline silicon is formed. Standard procedures for measuring the purity of high-purity silicon require a monocrystalline sample. This requirement presents problems when the polycrystalline silicon is in a particulate form.
The Czochralski method is a well known method for forming monocrystalline silicon in which particulate silicon is added to a heated quartz crucible to form a melt. A seed crystal is then contacted with the melt and slowly withdrawn to create a monocrystalline silicon rod. The problem with this technique of growing a monocrystalline silicon rod is that the silicon typically contains significant amounts of oxygen, carbon, boron, and other metallic impurities from the dissolution of some of the crucible wall during the crystal growth.
Rummel, U.S. Pat. No. 2,907,642, issued Oct. 6, 1959, describes a process whereby powdered silicon is blown into a melt zone to be incorporated therein and contribute to the formation of a monocrystalline silicon rod.
Emeis, U.S. Pat. No. 2,930,098, issued Mar. 29, 1960, proposed preparing particulate samples for possible float-zone treatment by sintering the particles in a quartz tube. The lower temperature required for sintering is reported to reduce contamination of the silicon due to deterioration of the quartz tube.
Imber, U.S. Pat. No. 3,156,533, issued Nov. 10, 1964, describes a process for converting powdered silicon into monocrystalline silicon with minimal contamination. The described invention relates to an apparatus for growing crystals which provides a conical crucible supported within a cylindrical chamber, and a powder dispensing means, or hopper suspended in the upper part of the chamber directly above the conically shaped crucible. A discharge opening in the apex of the conically-shaped crucible communicates with the lower part of the chamber where a seed crystal is supported directly beneath the opening. In operating the device, powder spills from the hopper onto the hot conical crucible where the powder melts and collects in a molten pool over the discharge opening of the cone. Drops form and fall through the opening to the crystal grown below.
None of the described procedures are entirely satisfactory for converting particulate silicon into monocrystalline silicon suitable for contaminates analysis. Processes which use a crucible to hold molten silicon result in contamination of the silicon, while non-crucible type processes are complicated and difficult to control. Therefore, it is an objective of the present invention to provide a method for float-zone processing of particulate silicon. It is a further objective to provide a method which reduces contamination during the conversion of polycrystalline silicon particles to monocrystalline, monolithic silicon. It is also an objective of the instant invention to provide a method, for determining contaminate levels of particulate silicon samples, which is reproducible and can be performed by means of standard equipment currently employed in float-zone processing.